Annealed porous polyolefin material

Abstract

A polyolefin material that comprises a thermoplastic composition that is annealed and thereafter drawn in a solid state is provided. The composition contains a continuous phase that includes a polyolefin matrix polymer and a nanoinclusion additive dispersed within the continuous phase in the form of discrete domains. A porous network is defined within the thermoplastic composition that includes a plurality of nanopores, wherein the thermoplastic composition has a glass transition temperature of from about −20° C. to about 50° C. as determined in accordance with ASTM E1640-13.

What is claimed is: 
     
       1. A polyolefin material that comprises a thermoplastic composition that is annealed and thereafter drawn in a solid state, the composition containing a continuous phase that includes a polyolefin matrix polymer and a nanoinclusion additive dispersed within the continuous phase in the form of discrete domains, wherein the nanoinclusion additive includes a polar component and a nonpolar component, wherein a porous network is defined within the thermoplastic composition that includes a plurality of nanopores, wherein the thermoplastic composition has a glass transition temperature of from −20° C. to 50° C. as determined in accordance with ASTM E1640-13. 
     
     
       2. The polyolefin material of  claim 1 , wherein the nanopores have an average cross-sectional dimension of 800 nanometers or less. 
     
     
       3. The polyolefin material of  claim 1 , wherein the composition has a density of 0.90 g/cm 3  or less. 
     
     
       4. The polyolefin material of  claim 1 , wherein nanopores have an average axial dimension of from 100 to 5000 nanometers. 
     
     
       5. The polyolefin material of  claim 1 , wherein the polyolefin matrix polymer has a melt flow rate of from 0.5 to 80 grams per 10 minutes as determined at a load of 2160 grams and at 230° C. in accordance with ASTM D1238. 
     
     
       6. The polyolefin material of  claim 1 , wherein the polyolefin matrix polymer is a substantially isotactic polypropylene homopolymer or a copolymer containing at least 90% by weight propylene. 
     
     
       7. The polyolefin material of  claim 1 , wherein the continuous phase constitutes from 60 wt. % to 99 wt. % of the thermoplastic composition. 
     
     
       8. The polyolefin material of  claim 1 , wherein the nanoinclusion additive includes a functionalized polyolefin. 
     
     
       9. The polyolefin material of  claim 8 , wherein the functionalized polyolefin is a polyepoxide. 
     
     
       10. The polyolefin material of  claim 1 , wherein the nanoinclusion additive includes a polymer having a melt flow rate of from 0.1 to 100 grams per 10 minutes as determined at a load of 2160 grams and at a temperature at least 40° C. above the melting temperature in accordance with ASTM D1238. 
     
     
       11. The polyolefin material of  claim 10 , wherein the ratio of the melt flow rate of the polyolefin to the melt flow rate of the nanoinclusion additive is from 0.2 to 8. 
     
     
       12. The polyolefin material of  claim 1 , wherein the nanoinclusion additive is in the form of nano-scale domains, wherein the nano-scale domains have an average cross-sectional dimension of from 1 nanometer to 1000 nanometers. 
     
     
       13. The polyolefin material of  claim 1 , where in the nanoinclusion additive constitutes from 0.05 wt. % to 20 wt. % of the composition, based on the weight of the continuous phase. 
     
     
       14. The polyolefin material of  claim 1 , wherein the composition further comprises a microinclusion additive dispersed within the continuous phase in the form of discrete domains. 
     
     
       15. The polyolefin material of  claim 1 , wherein the thermoplastic composition further comprises an interphase modifier. 
     
     
       16. The polyolefin material of  claim 1 , wherein the porous network further includes micropores. 
     
     
       17. The polyolefin material of  claim 1 , wherein the total pore volume of the polyolefin material is from 15% to 80% per cubic centimeter. 
     
     
       18. The polyolefin material of  claim 1 , wherein nanopores constitute 20 vol. % or more of the total pore volume of the polyolefin material. 
     
     
       19. A fiber comprising the polyolefin material of  claim 1 . 
     
     
       20. A nonwoven web comprising the fiber of  claim 19 . 
     
     
       21. An absorbent article that includes a substantially liquid-impermeable layer, liquid-permeable layer, and an absorbent core, wherein the substantially liquid-impermeable layer, the liquid-permeable layer, or both include the polyolefin material of  claim 1 . 
     
     
       22. A method for forming a polyolefin material, the method comprising:
 forming a thermoplastic composition that contains a continuous phase that includes a polyolefin matrix polymer and a nanoinclusion additive dispersed within the continuous phase in the form of discrete domains, wherein the nanoinclusion additive includes a polar component and a nonpolar component; 
 annealing the thermoplastic composition; and 
 solid state drawing the annealed thermoplastic composition to form a porous network therein, the porous network including a plurality of nanopores; 
 wherein the thermoplastic composition has a glass transition temperature of from −20° C. to 50° C. as determined in accordance with ASTM E1640-13. 
 
     
     
       23. The method of  claim 22 , wherein annealing occurs at a temperature of from 50° C. to 160° C. 
     
     
       24. The method of  claim 22 , wherein annealing occurs for a time of from 5 to 200 minutes. 
     
     
       25. The method of  claim 22 , wherein the thermoplastic composition is drawn to a stretch ratio of from 1.1 to 25. 
     
     
       26. The method of  claim 22 , wherein the thermoplastic composition is drawn at a temperature of from −50° C. to 150° C.